The indeterminate phase of Chagas' disease is defined as the prolonged period of clinically silent infection that follows the phase of acute primary infection with Trypanosoma cruzi. The dog is the only experimental animal model in which the indeterminate phase progresses to the late phase of severe, chronic myocarditis. This report describes the cardiac histologic and ultrastructural findings in dogs that survived the acute phase of infection with T. cruzi, becoming clinically and electrocardiographically normal for up to 3 1/2 years, while maintaining positive serological tests during this period of time. Most of the myocardium appeared morphologically normal; however, small foci of mild, chronic myocarditis were present, with interstitial edema, mild fibrosis and infiltration by lymphocytes, macrophages and plasma cells. No microvascular lesions and no areas of close contact between immune effector cells and endothelial cells or cardiac myocytes were present. These findings were in sharp contrast to those observed in the canine model during the acute infection with T. cruzi. In this model, acute myocyte damage and lesions in the microcirculation, including fibrin microthrombi, were associated with close contacts between immune effector cells and myocytes or endothelial cells. Focally inflamed interstitial tissue showed increased deposition of amorphous and collagenous extracellular matrix as well as evidence of breakdown of collagen. The features of the inflammatory cells in the indeterminate phase of Chagas' disease were interpreted as indicating a self-limited cycle of focal inflammatory changes, with modulation and suppression of cell-mediated immune responses. Thus, we consider the indeterminate phase of Chagas' disease to be a stage of host-parasite equilibrium rather than a process of progressive damage.